The invention relates to a particle beam apparatus with a source for providing a primary particle beam along a primary beam axis, an objective lens for focussing the primary particle beam onto a specimen and a detection system for image generation.
As integrated circuits are fabricated with features of less than 0.1 xcexcm, electron beam imaging has been established as the technology of choice for process development and quality assurance. However, some specimens, especially those containing features having a high aspect ratio, as, for example, contact holes having a diameter of only 0.1 xcexcm and a depth of 1 xcexcm are very difficult to inspect during manufacturing. Such a contact hole has an aspect ratio of 10:1 and, accordingly, it is very difficult to get a signal from the interior of such a feature in order to create an image of the lower part of the high aspect ratio feature.
In the state of the art high resolution objective lenses having electrostatic retarding field lenses or combined electromagnet lenses are applied. Such lenses use higher beam energies in front of the lens and decelerate the primary beam to the lower final energy inside the objective lens. The deceleration of the primary beam is also used for extraction and acceleration of the particles released at the specimen. The released particles are transferred to the detection means inside or in front of the lens for registration and for forming an image of the specimen""s surface using the secondary electrons released at the specimen.
A drawback of such an arrangement is the hole in the detector, which is required for the penetration of the primary particle beam. Since scintillation detectors are built from insulating material, this hole has to have a certain size in order to avoid charging. Secondary electrons with starting angles around 90xc2x0 just meet the detector in its hole area and get lost. For features having a high aspect ratio, the signal electrons carrying the relevant information are exactly those secondary electrons. Consequently, the inner part of those high aspect features can be imaged only with small signal amplitude, which means a poor signal to noise ratio and consequently a limited information.
To overcome this drawback, it has been proposed to apply a beam separator for separating the primary beam and the secondary electron beam. This enables the application of a detector without a hole. The beam separator, however, is an additional component, which introduces aberrations to the primary particle beam and consequently, the system resolution is negatively affected.
It was also suggested to provide a deceleration area in front of the objective lens. In this area the secondary electrons are so slow that they easily can be gathered. This also enables the detection of the xe2x80x9con-axisxe2x80x9d secondary electrons. This approach, however, requires an additional lens which generates an additional crossover in the beam path which increases the energy width of the primary beam and may negatively effect the resolution.
U.S. Pat. No. 5,644,132 discloses a particle beam apparatus for charge-free high resolution imaging and measurement of topographic and material features on a specimen. A particle source provides a primary beam along a primary beam axis, the primary beam impinging on the specimen so as to release electrons therefrom. The electrons includes secondary electrons and backscattered electrons. An objective lens is focusing electrons so as to provide a radial dispersion of the electrons relative to the primary beam axis, the radial dispersion of electrons including an inner annulus of backscattered electrons and an outer annulus of secondary electrons. Furthermore, the apparatus comprises a backscattered electron detector for detecting the inner annulus of backscattered electrons and a secondary electron detector for detecting the outer annulus of secondary electrons. The backscattered electron detector is an electron multiplier.
To reduce the negative influence of the detector hole, the detection of near-axial backscattered electrons is preferably enhanced by forming a flange termination from material which facilitates the generation of secondary electrons by impacting backscattered electrons. These secondary electrons can be additionally detected as described in U.S. Pat. No. 5,466,940.
This known detection system also includes a generally annular electrode for ensuring that the backscattered electron detector detects not only those backscattered electrons which impinge on the active area of the electron multiplier, but also those backscattered electrons which impinge on the inactive area of the electron multiplier and those backscattered electrons which impinge on the flange termination and are converted into secondary particles.
U.S. Pat. No. 5,466,940 further discloses a configuration in which the electron detector is deployed within the optical column. The objective lens of this apparatus comprises an electromagnetic lens which accelerates secondary electrons released from the specimen towards the electron detector.
U.S. Pat. No. 4,308,457 discloses a device for the detection of back-scattered electrons emitted by a specimen in an electron microscope, comprising a converter for converting back-scattered electrons emitted by the specimen into secondary electrons.
It is an object of the invention to provide a particle beam apparatus according to the present invention having an enhanced contrast information with a simple arrangement.
According to the invention, the particle beam apparatus comprises:
a source for providing a primary particle beam along a primary beam axis,
an objective lens for focussing the primary particle beam onto a specimen so as to release particles therefrom and
a detection system for image generation, the detection system comprises:
converter means with a conversion area to convert the released accelerated particles into secondary particles,
electrode means for influencing the converted secondary particles, and
at least one detector for detecting the converted secondary particles,
wherein the objective lens (4) is formed by an immersion lens for decelerating the primary particle beam from a first higher energy to a second final energy before the primary particle beam impinges on the specimen (5) so as to release particles (6) therefrom, the released particles being accelerated by the immersion lens before reaching the detector system (7).
The converter means and the electrode means are adapted to control the converted secondary particles in that a suitable voltage between the converter means and the electrode means prevents converted secondary particles released at a specific part or parts of the conversion area from reaching the detector.
The converter means are preferably formed by a converter plate having at least one opening for the primary particle beam. The converter plate is usually made of conductive material, the opening required for the primary particle beam may be very small, typically less than 500 xcexcm. Therefore, the loss of particles released at the specimen in this region is reduced significantly.
In order to improve the contrast information, it is necessary to differentiate between locations on which the released particles impinge on the converter means or, in other words, to evaluate the signal of those particles that are converted at a specific part or parts of the conversion area. An interesting part of the conversion area may be an inner or outer annular area of the converter means. The different parts of the conversion area may also have the form of annular segments.
In the prior art the contrast information is obtained by using the different detectors. According to the present invention, this information will be obtained in that the converter means and the electrode means are adapted to control the converted secondary particles in that a suitable voltage between the converter means and the electrode means prevents the converted secondary particles released at a specific part or parts of the conversion area from reaching the detector.